Field of the Invention
The invention concerns the fabrication of plates or blanks of coated steel intended to be welded and then heat treated to obtain parts having good mechanical characteristics and good corrosion resistance.
Some applications require steel parts combining high mechanical strength, high impact resistance and good corrosion resistance. This type of combination is particularly desirable in the automotive industry which requires a significant reduction in vehicle weight and excellent capacity to absorb energy in the event of a collision. This can be achieved in particular by using steel with very good mechanical characteristics having a martensitic or bainitic-martensitic microstructure: anti-intrusion, structural or safety components of automotive vehicles such as bumpers, door reinforcements, B-pillar reinforcements or roof reinforcements, for example, require the above qualities.
Description of the Related Art
Patent EP 0971044 discloses a fabrication method in which hot- or cold-rolled steel plate coated with aluminum of aluminum alloy is the starting material. After shaping to produce a part, and before heat treatment at a temperature above Ac1, the coating is heated to form a surface alloy by interdiffusion between the steel and the aluminum coating. This alloy prevents decarburization of the metal and oxidation during heat treatment in a furnace. It therefore eliminates the necessity for furnaces containing a special atmosphere. The presence of this alloy also obviates certain surface operations on the treated parts, such as shot blasting, which operations are necessary for plates having no coating. The parts are then cooled under conditions adapted to confer a tensile strength that can exceed 1500 MPa.
With the aim of reducing vehicle weights, parts have been developed consisting of steel blanks of different compositions or different thicknesses continuously butt-welded together. These welded parts are known as “butt-welded blanks”. Laser beam welding is a preferred method of assembling such blanks, exploiting the flexibility, quality and productivity characteristics of the process. After these welded blanks have been cold-pressed, parts are obtained having mechanical strength, pressability, impact absorption properties that vary within the parts themselves. It is therefore possible to provide the required properties at the appropriate location without imposing an unnecessary or costly penalty on all of the parts.
The fabrication method described in patent EP 0971044 can be applied to butt-welded blanks in the following manner: starting from steel plate, possibly of different compositions or thicknesses, and having a metal pre-coating, butt-welded blanks are obtained by a welding process. These welded blanks then undergo heat treatment to form a surface alloy and are then hot-pressed and quenched. This produces quenched parts with thicknesses and intrinsic mechanical characteristics that vary and represent an ideal response to local loading requirements.
However, this fabrication method runs into considerable difficulties: when welding coated steel blanks, a portion of the initial surface pre-coating is transferred into the molten area created by the welding operation. These exogenous metal elements are concentrated in particular by strong convection currents in the liquid metal. These elements are segregated in particular in the interdendritic spaces in which the liquid fraction having the greatest concentration of dissolved elements is located. If austenizing follows with a view to quenching the welded blanks, these enriched areas become alloyed through interdiffusion with the iron or other elements of the matrix and form intermetallic areas. On subsequent mechanical loading, these intermetallic areas tend to be the site of onset of rupture under static or dynamic conditions. The overall deformability of the welded joints after heat treatment is therefore significantly reduced by the presence of these intermetallic areas resulting from welding and subsequent alloying and austenizing.
It is therefore desirable to eliminate the source of these intermetallic areas, namely the initial surface metal coating liable to be melted during butt-welding. However, eliminating this source itself gives rise to a serious problem: the precoated area on either side of the future welded joint can be eliminated, for example a mechanical process. The width of this area from which the pre-coating is removed must be at least equal to that of the future area melted by welding so as not to encourage subsequent formation of intermetallic areas. In practice, it must be much more than this, to allow for fluctuations in the width of the molten area during the assembly operation. Thus there exist after the welding operation areas on either side of the welded joint that no longer have any surface metal pre-coating. During further alloying and austenizing heat treatment, scale formation and decarburizing occur within these areas located next to the weld. These are areas that tend to corrode when the parts go into service because they are not protected by any coating.
There is therefore a requirement for a fabrication process that prevents the formation of intermetallic areas within welded assemblies, which are sources of the onset of rupture.
There is also a requirement for a fabrication process such that the welded and heat treated parts have good corrosion resistance.
There is also a requirement for an economic fabrication process that can be integrated without difficulty into existing welding lines and that is compatible with subsequent pressing or heat treatment phases.
There is also a requirement for a product on which operations of butt-welding, then of heat treatment, pressing and quenching, lead to the fabrication of a part having satisfactory ductility and good corrosion resistance. One particular requirement is for a total elongation across the welded joint greater than or equal to 4%.
An object of the present invention is to solve the problems referred to above.